Four channel stereo synthesizer

ABSTRACT

A stereo synthesizer according to which a four-channel &#39;&#39;&#39;&#39;surround stereo&#39;&#39;&#39;&#39; is developed from either two-channel or single-channel sources. The system performs a number of transformations on the source material, and the resulting signals are reproduced through two sets of loudspeakers, one set of these loudspeakers being located in front of the listener and the other set of loudspeakers being located behind the listener. As a consequence if this arrangement, the listener is afforded a sense of space and dimension which is superior to the sensation produced through one or two loudspeakers.

United States Patent Orban [4 1 Aug. 15, 1972 [54] FOUR CHANNEL STEREO SYNTHESIZER Primary Examiner--Kathleen l-l. Claffy 72 Inventor: Robert Arnold Orban Menlo Park, Assistant ExMMH'JOm D'Amico Calif. Attorney-Robert S. Dunham, P. E. Henninger, Lester W. Clark, Gerald W. Griffin, 'llhomas F. Moran, R. [731 Asslgmfl M Inc-r San Franclscor Bradlee Boal, Christopher c. Dunham, Howard J. Cahf- Churchill, Robert Scobey, Henry T. Burke and John [22] Filed: July 29, 1970 F. Ohlandt, Jr.

A stereo synthesizer according to which a four-chan- C(il ..l79/l Gkggillgg nel usunourld stereo" is developed from either [58] Field of Search 179/1 0, l GA, 16?, 15 BT channel mums- The system performs a number of transformations on Referfllces Cited the source material, and the resulting signals are reproduced through two sets of loudspeakers, one set UNITED STATES PATENTS of these loudspeakers being located in front of the 2,819,342 l/l958 Becker ..l79/l G listener and the other set of loudspeakers being 2,845,491 7/ B r ram G located behind the listener. As a consequence if this PCl'ClVfil e. G arrangement the listener is afi'orded a sense of pace Fouq-ue G and dimension is uperior to the sensation 3,478,167 1 H1969 Sorkln I 79/1 GA produced through one or two |oudspeakers 2,768,237 10/1956 Faulkner ..l79/l G 3,087,988 4/1963 Somer l 79/l GP 4 Claims, 5 Drawing Figures 40 REVERE 651/5?!) 7 0A 5 TZKE'O A M/ our 20 (22 Z SYA/Tl/ fS/Zfl? 5 S I 24 54K STEREO 33 PM M/PUTF f0 30 Yd Q/MD/PAIURE 7 Gill/f'kflM/Q Y FAOA/T 3 42 FOUR CHANNEL STEREO SYNTHESIZER BACKGROUND, OBJECTS & SUMMARY OF THE INVENTION The invention of the present application is related to 'the invention disclosed in copending application, Ser.

No. 25,775, filed Apr. 6, I970, which application is a continuation-in-part of application, Ser. No. 667,216, filed Sept. 12, 1967, now abandoned.

The invention of the aforesaid earlier filed application describes a stereo synthesizer system that is especially adapted for converting monophonic, i.e. singlechannel, sound signals into stereophonic sound signals so that the signals so converted may be reproduced and presented in a variety of forms. In accordance with the stereo synthesizer system of the earlier filed application, the source of monophonic sound signals is connected to two independent sound channels; moreover, the signals are applied directly to the two sound channels, and also in delayed fashion thereto, by means of a special network transfer function, whereby the direct and delayed signals are combined so as to provide reinforcement in one channel and to be opposed in another channel as a function of frequency. The specialized nature or character of the network transfer function is such that the amplitude of the frequency response is independent of frequency; in other words, there is a flat frequency response. However, the phase response is readily varied as a function of frequency; in other words, different frequency bands have their corresponding delayed signals independently adjusted in accordance with their position in the frequency spectrum.

The fundamental advantage of the earlier invention is that the phase shifting arrangement described above is very flexible and economical. Moreover, it is electrical in nature, is extremely effective in the short delay range, and as noted above, can be widely adjusted throughout the frequency spectrum.

The essential components of the network transfer function mentioned previously include a processor in which a succession or cascade of individual phase shift networks are provided in order to produce a readily adjustable set of cross-over points in the range or spectrum of sound frequencies. Another component of the network transfer function is a matrixing means which operates on the output as provided by the phase shift networks, and suitably combines the direct signals with the delayed signals that have been operated upon by the processor.

Although the stereo synthesizer of application, Ser. No. 25,775 has been found useful for a great number of applications involving two output devices, such as two loudspeakers appropriately spaced in front of a listener, it has been found that an improved sense of space can be achieved by reorganizing the stereo synthesizer system to produce an output having several speakers located in front of the listener, such speakers being fed by one group of signals and by having another group of speakers at the rear of the listener, the latter group having supplied to them signals differing from those applied to the first group. Accordingly, in an illustrative embodiment of the present invention, the output from a stereo synthesizer, as described in application Ser. No. 25,775 is applied to the front pair of speakers. This kind of arrangement is especially adapted to a situation in which only a monophonic source of sound is to be utilized. However, the principle of the present invention can also be applied to systems in which a stereophonic source is available and is to be utilized; that is to say, plural independent channels or inputs are available, as will be made clear hereinafter. In the latter case, the stereo inputs supplied to the front pair of speakers are also combined for application as if they were a monophonic source and are then operated upon and connected to a rear group of speakers. In this way the listener is afforded a sense of space and dimension which is superior to that when only a front group of speakers are employed.

The feature of the present invention which makes the aforesaid possible is the provision of a phase quadrature generator, per se well-known, which is so arranged to produce widely diffused, yet non-directional sound from the rear speakers. The quadrature generator, which is a pair of phase-shifters so arranged as to give a constant phase difference of between their outputs throughout the audible range of 20-20,000 Hz, is fed by a combination of the direct signal emanating from the front speakers and the same signal processed through a reverberation generator. The reverberation generator provides a simulation of the natural reverberation in a large hall. However, more direct sound than reverberated sound is introduced to the rear speakers in order to avoid seriously upsetting the ratio of a direct to reverberated sound existing in the original recording.

Because of the nature of the quadrature generator, there exist large, frequency-dependent phase-shifts between the signal appearing in the front speakers and the direct component of the signal appearing in the rear speakers. In addition, the reverberative component has a frequency-independent time delay with respect to the front speakers. These phase shifts and delays also help introduce a sense of large space into the four-channel reproduction. In combination with the diffusion of the rear information caused by the quadrature generator, these characteristics of the present invention create a sense of overall acoustical environment.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram illustrating the listening set-up for the enjoyment of the system of the present invention.

FIG. 2 is a functional diagram of the system of the present invention according to one embodiment in which the monophonic source and the two channels connected to the front speakers are provided by utilizing the stereo synthesizer in accordance with a prior invention.

FIG. 3 is a functional diagram of another embodiment in which a stereophonic source or inputs exist, and one set of speakers are connected directly to such stereophonic source. 7

FIG. 4 is a schematic diagram of the phase quadrature generator as utilized in the system of the present invention.

FIG. 5 is a functional diagram partly in block form, illustrating the interconnections in order to provide either mode or embodiment of the present invention, that is, to translate from either a monophonic source or from a two-channel stereo source, to a four-channel output.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawing and for the moment FIG. 1, there is shown a typical listening set-up to be used in connection with the present invention. In this set-up the listener is surrounded by a number of loudspeakers, two of which are normally placed to the front and connected in a manner to be described. Another set of speakers is placed to the rear of the listener and their output is varied from that supplied to the front speakers.

Referring now to FIG. 2, there is shown an illustrative embodiment in which a stereophonic sound source, comprising two independent inputs, is available and is to be suitable connected to produce a four-channel output system according to the present invention.

Starting with stereo inputs as indicated on the drawing, suitable leads are connected to a pair of loudspeakers designated front pair and the inputs are ap plied to a typical combining or summing device for the purpose of converting to a monophonic input source. Thence, from the output of the device 10, the single output is taken to two independent channels in each of which a special function is provided. In the upper channel, designated 20, a simulation of the reverberation ordinarily encountered in a large concert hall is achieved by a suitable reverberation generator 22, the output of which is connected through a potentiometer 24 and thence to a stereo synthesizer 26.

The stereo synthesizer 26 is of the'type already described in previously filed application, Ser. No. 25,775, in the name of the present inventor. The detailed description of the stereo synthesizer is incorporated herein by reference, such detailed description being unnecessary for an understanding of the present invention. However, it should be noted that such a stereo synthesizer essentially comprises an arrangement for synthesizing a stereo output from a monophonic input, according to which the source of monophonic sound signals is connected to two independent sound channels, the signals being applied directly to the two sound channels, and also in delayed fashion thereto, by means of a network transfer function so as to reinforce the direct signals in one channel, and oppose them in another channel as a function of frequency. The network transfer function is such that the amplitude of the frequency response is independent of frequency while the phase response varies as a function of frequency. As a consequence of such an arrangement, the very slight phase shifts that are effected by the network transfer function realize the desirable feeling normally associated with true stereophonic sound reproduction.

In the lower channel 30, another potentiometer 32 is provided at the input to quadrature generator 34. This quadrature generator, which will be described hereinafter, enables the generation of widely diffused yet non-directional sounds. Essentially, it consists of two all-pass networks whose time constants are staggered in such a way that the phase difference between the outputs of the networks is closely equal to 90 throughout the usual audible range of 20,000 Hz.

The two outputs from the quadrature generator 34 are combined with the two outputs from the stereo synthesizer previously explained in a pair of summing devices 40 and 42. The output from each of the summing devices is connected to a loudspeaker. This pair or set of speakers is usually located to the rear of the listener.

It has been found in the practice of the invention according to the illustrative embodiment just described that there is far more quadrature split signal than reverberated signal in the mix emanating from the rear loudspeakers. However, some reverberation is always necessary to obtain the desired effect.

It should be noted that the stereo synthesizer 26 may be eliminated. In this case, the output from the reverberation generator 22 may be connected as indicated by dotted line 33, and the reverberated signal may be processed through the quadrature generator 34 along with the direct signal.

Referring now to FIG. 3, another embodiment of the principle of the present invention is there illustrated. In this embodiment, stereophonic inputs are not involved but only a monophonic input, such monophonic input being connected in one channel by way of line 52 to a stereo synthesizer 54 which is another piece of equipment of the type already noted. As a consequence, the monophonic input is converted or translated into a simulated stereophonic output which is applied to a set of loudspeakers designated front pair."

The monophonic input is also applied in parallel to a two channel arrangement identical to that previously shown in FIG. 2, the parts of which have been designated with identical numerals. Also, several alternate connections can be provided in connection with the FIG. 3. embodiment, as was the case previously with the embodiment of FIG. 2. Thus, the stereo synthesizer 26 may be eliminated, and the output from the reverberation generator 22 may be processed through the quadrature generator 34 by way of line 33. In addition, other changes may be made: The stereo synthesizer 54 may be eliminated and both lines to the front pair of speakers may be driven directly; or the stereo synthesizer 54 can be replaced by a wide band phase-shifter or other pseudo-stereo generator.

Referring now to FIG. 4, a schematic diagram is provided of the quadrature generator 34. This generator consists essentially of two all-pass networks arranged so that the phase difference of their outputs is approximately at frequencies from 20-20,000 Hz.

Starting at the monophonic input 60, the input is applied in common to an upper channel, designated 62, and a lower channel, designated 64. In each of the upper and lower channels a series of cascaded phase shifting networks is arranged. The coupling capacitors 66 and 68 at the individual inputs to the respective channels are provided for AC/DC isolation. It will be noted that the D.C. bias for the whole circuit shown in FIG. 4 is determined by the 200K] K voltage dividers 70 and 72. Further, it will be appreciated that each of the stages in the upper and lower channels consists of pairs of transistors. Thus the upper channel is defined by the five stages comprising the pairs Ql-Q2 through 09-010 and similarly, the lower channel is defined by the transistor pairs Qll-Q12 through 019-020. Each of the aforesaid stages possesses almost exactly unity gain from its input to its output.

Since the various stages of the phase quadrature generator of FIG. 4 are very similar except for the values of capacitors in each of the stages, individual numerals have not been assigned to the components;

rather, the values of such components are given in parentheses. All of the resistor values are given in ohms; whereas, capacitor valves, except where indicated, are given in microforads. It will be seen from FIG. 4 that for the successive stages, capacitor values are in decreasing order, the time constants having been chosen to ensure that the phase difference between the outputs will be approximately 90.

As will be appreciated, the separate outputs from the quadrature generator 34 are applied, together with the A and B outputs from the stereo synthesizer 26, to the summing network 40 and 42, whereby the desired output signals to the rear pair of speakers is obtained.

Referring now to FIG. 5, an interconnection arrangement is illustrated which enables selection of either mode of operation as illustrated in FIG. 2 and FIG. 3.

Considering the first mode of operation, whereby stereophonic input signals appear at both of the terminals, L-stereo-in and R-stereo-in, such signals are applied by way of lines 100 and l02to the several fixed contacts of the switch 104, whereby these signals may be transmitted to the front speakers by proper positioning of the movable switch contacts 104x and l04y, i.e. to either the left or center position.

The stereo input signals are also applied to the summing amplifier 110, thus implementing the functional block previously shown in FIG. 2. The output of summing amplifier 110 is applied to both of the channels and 30, the outputs of each of the channels being applied conjointly to the input of emitter-follower 120 and 122 so as to implement the summing function performed by the functional blocks 40 and 42 of FIG. 2 The equipment shown in channels 20 and 30 is, of course, identical with that previously described.

The output signals from the emitter-followers 120 and 122 are transmitted by way of the lines 130 and 132 respectively, to the fixed contacts of the switch 134, i.e. contacts 134f and c and 134s and f, whence they are transmitted to the rear group of speakers by proper positioning of the movable contacts.

The stereo synthesizer 54 is used to operate upon a monophonic input when such an input appears at mono-in and is applied by way of line 150 to the input of that stereo synthesizer. In the manner already described, the stereo synthesizer provides two output channels in which direct and delayed signals are combined so as to provide reinforcement in one channel, and be opposed in another channel as a function of frequency. The outputs of the synthesizer 54 are connected to two amplifiers 56 whose outputs in turn are connected to the fixed switch contacts 1040 and f, which are the contacts appropriate to the monophonic mode of operation.

It will be appreciated, of course, that when it is desired to apply signals directly to the front-and rear speakers without utilization of the equipment shown in FIG. 5, a bypass position is provided for the switches 104 and 134, according to which the movable contacts are thrown to their farthest left position.

While there have been shown and described and pointed out the fundamental novel features of the invention as applied to the preferred embodiments, it will be understood that various omissions and substitutions and changes in the form and details of the device illustrated and in its operation may be madeby those skilled In the art without departing from the spirit of the invention. It is the intention, therefore, to be limited only as indicated by the scope of the following claims.

What is claimed is: e I

l. A multi-channel stereo synthesizer apparatus for providing an acoustic output which completely surrounds a listener and creates a sense of space comprising:

a first group of sound reproducers in a first location adjacent the listener,

a second group of sound reproducers in a location adjacent said listener and opposite said first location,

means for supplying stereophonically related signals to said first group of sound reproducers,

means for supplying signals, different from said stereophonically related signals, to said second group of sound reproducers including a. a pair of independent channels having their outputs connected to said second group of sound reproducers,

b. means for providing monophonic sound signals at the inputs to each of said channels,

c. a quadrature generator connected in one of said pair of channels, said quadrature generator including two all-pass networks having outputs differing in phase by approximately, at frequencies from 20 20,000 Hz, said outputs being connected to different ones of said second group of sound reproducers.

' 2. Apparatus as defined in claim 1, further including a reverberation generator connected in the other one of said pair of channels.

3. Apparatus as defined in claim 1, further including a stereo synthesizer in said other channel, said stereo synthesizer having two outputs and a single input, the signals from said input being processed such that direct and delayed signals are combined so as to provide rein' forcement thereof at one of the outputs and opposition thereof at the other output as a function of frequency.

4. Apparatus as defined in claim 3, further including a summing network in each of said channels, one of the outputs of said stereo synthesizer and of said quadrature generator being directly connected to the summing network in said one channel, and the other output of said stereo synthesizer and of said quadrature generator being connected to the summing network in the other channel. 

1. A multi-channel stereo synthesizer apparatus for providing an acoustic output which completely surrounds a listener and creates a sense of space comprising: a first group of sound reproducers in a first location adjacent the listener, a second group of sound reproducers in a location adjacent said listener and opposite said first location, means for supplying stereophonically related signals to said first group of sound reproducers, means for supplying signals, different from said stereophonically related signals, to said second group of sound reproducers including a. a pair of independent channels having their outputs connected to said second group of sound reproducers, b. means for providing monophonic sound signals at the inputs to each of said channels, c. a quadrature generator connected in one of said pair of channels, said quadrature generator including two all-pass networks having outputs differing in phase by approximately 90* at frequencies from 20 - 20,000 Hz, said outputs being connected to different ones of said second group of sound reproducers.
 2. Apparatus as defined in claim 1, further including a reverberation generator connected in the other one of said pair of channels.
 3. Apparatus as defined in claim 1, further including a stereo synthesizer in said other channel, said stereo synthesizer having two outputs and a single input, the signals from said input being processed such that direct and delayed signals are combined so as to provide reinforcement thereof at one of the outputs and opposition thereof at the other output as a function of frequency.
 4. Apparatus as defined in claim 3, further including a summing network in each of said channels, one of the outputs of said stereo synthesizer and of said quadrature generator being directly connected to the summing network in said one channel, and the other output of said stereo synthesizer and of said quadrature generator being connected to the summing network in the other channel. 